Switch to go mod and update some dependencies

[mihaitodor]

Hey @relistan I made an experiment with go mod in this repo before doing Sidecar. Seems to be more or less straight-forward, although it does expect to be able to fetch the vendored dependencies from the internet during the migration. In this case, it went fine.

Other changes:

• Update Travis Go version to 1.12
• Convert Sirupsen/logrus to sirupsen/logrus

[relistan]

Sounds good, but why are there 679k fewer lines of code in here? Do the modules contain that much less stuff?

[mihaitodor]

I was surprised by that as well. Looks like go mod removes a lot of stuff when vendoring, such as tests. I have mixed feelings about that, but, apparently, it's by design (see here a comment from Russ Cox).

According to the docs setting GOFLAGS=-mod=vendor (like I did in .travis) instructs the build tool to not download dependencies from the internet, so all the dependencies need to be present in the vendor folder or, otherwise, it will fail to build:

The -mod=vendor flag (e.g., go build -mod=vendor) instructs the go commands to use the main module's top-level vendor directory to satisfy dependencies. The go commands in this mode therefore ignore the dependency descriptions in go.mod and assume that the vendor directory holds the correct copies of dependencies. Note that only the main module's top-level vendor directory is used; vendor directories in other locations are still ignored.

[relistan]